In many cases, saving a page locally without the linked fonts will
produce poor results. First of all, the Unicode Standard includes a huge
number of interesting characters that are currently not being included
as text in web pages, but could, when the use of webfonts increases.
For example arrows, box drawing, block elements, geometric shapes,
symbols, dingbats, or the newly-added emoticons could be easily used on
web pages, especially with webfonts. Some of those characters may be
very well not included at all in any system fonts, so webfonts may be
the only means to visualize those.
The same would apply for texts written in some less-known or minority
writing systems.
If the fonts are not included, the text on the page becomes completely
unreadable.
Also, some webpage layouts will completely explode if the original font
is not used. If I design a page using a very condensed font at a
reasonably large size, and fit many columns into the page width, the
page may look great when the webfonts are available, but if they are
not, the complete page will "fall apart".
HTML is, in the end, more of a page layout language than just a text
markup scheme. Preserving the typographic appearance when the page is
saved locally is just as important as preserving images.
When we're trying to educate web developers that they should adopt
webfonts because this way they can stop using "text as image"
techniques, we need to make sure that this switch won't backfire on
them. Nothing will be more off-putting than finding out that, once the
page has been saved locally (or printed, for that matter), the page has
been rendered useless because characters would appear as .notdefs rather
than the intended glyphs.
Since the early 1990s, computer users are used to WYSIWYG. When I print
a Word document, I expect the prinout to closely resemble the screen
appearance, with the changes necessary for switching the media (i.e. the
text would be higher-resolution when printed than on screen, or images
would be grayscale if printed on a monochrome printer). I think users
expect the same to be the case with the web.
I doubt any user will understand why a page that looks fine on the
screen should, when saved or printed, appear crippled. Should that
happen, web users will complain to web developers, and those might, in
the end, turn webfonts their backs by deeming them "unreliable technology".
Best,
Adam